The Shredder

Because I have a Mac, finding compatible apps is always bit of a challenge.  Thankfully there's one program I found to be pretty workable called "Shredder" (no relation to the TMNT villian).  It's pretty simple and easy to use.  Full Elo rating adjustment and all that jazz.  A good offline chess app for the more casual chess player

You can find it at: http://www.shredderchess.com/